Simple Synthesis of Large Graphene Oxide Sheets via Electrochemical Method Coupled with Oxidation Process.

In this paper, we report a simple two-step approach for the synthesis of large graphene oxide (GO) sheets with lateral dimensions of ≈10 μm or greater. The first step is a pretreatment step involving electrochemical exfoliation of graphite electrode to produce graphene in a mixture of H2SO4 and H3PO4. The second step is the oxidation step, where oxidation of exfoliated graphene sheets was performed using KMnO4 as the oxidizing agent. The oxidation was carried out for different times ranging from 1 to 12 h at ∼60 °C. Prepared GO batches were characterized using a number of spectroscopy and microscopy techniques such as X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FTIR), and UV-visible spectroscopy. Raman and thermogravimetric analysis techniques were used to study the degree of oxidation in the as-synthesized GO batches. The UV-visible absorption spectrum showed an intense peak at 230 nm and an adjacent band at 300 nm corresponding to π-π* and n-π* transitions in all samples. Normalized FTIR plots were used to calculate the relative percentages of oxygen-containing functional groups, which were found to be maximum in GO (6 h). Boehm titration was used to quantify the functional groups present on the GO surface. Overall GO sheets obtained after 6 h of oxidation, GO (6 h), were found to be the best. XRD pattern of GO (6 h) revealed a characteristic peak at 2θ = 8.88°, with the corresponding interplanar spacing between the layers being 0.995 nm, which is among the best with respect to the previous methods reported in the literature. Raman spectroscopy showed that the degree of defect (I D/I G) area ratio for GO (6 h) was 1.24, which is higher than that obtained for GO (1.18) prepared by widely used Marcano's approach.

Introduction

Graphene is an emerging two-dimensional carbon material containing sp2 hybridized carbon atoms arranged in hexagonal array.[1−4] It is a future material due to its excellent physicochemical properties such as high surface area (2630 m2 g–1), thermal conductivity (5000 W m–1 K–1), Young’s modulus (1 TPa), electron mobility (2.5 × 105 cm2 V–1 s–1), relatively high electrical conductivity at room temperature (of the order of 106), and chemical durability.[5−10] Due to its extraordinary structure-related properties, it has found potential applications in various fields of science and engineering, including biotechnology, energy, and environment.[11−14] Some of the commonly used methods for the preparation of graphene include scotch tape method, chemical vapor deposition, liquid-phase exfoliation, electrochemical exfoliation, and chemical reduction of graphene oxide (GO). Among them, chemical reduction of GO to yield graphene sheets is the most widely used technique due to its cost-effectiveness and feasibility of processing. In this process, graphite is first oxidized to give GO nanosheets using Hummers’ method and then reduced to yield graphene. During oxidation, C=C double bonds break down and oxygen functional groups, such as −COOH, −OH, and C–O–C, are introduced in the basal plane and at the edges of the graphene sheets. These functional groups are further reduced by chemical, thermal, and electrochemical treatments.

The well-known Hummers’ method was introduced by Hummers and Offman in 1958, which made use of sodium nitrate (NaNO3) and potassium permanganate (KMnO4) in H2SO4 for performing oxidation of graphite flakes. Generation of toxic gases, residual nitrate content, and low yield are some of the major flaws of the Hummers’ process.[15−23] Later, several modifications have been proposed in the Hummers’ process to synthesize GO sheets with high degree of oxidation.[20,22,24] In 2010, Marcano and co-workers replaced NaNO3 with H3PO4. They concluded that GO prepared using KMnO4, H2SO4, and H3PO4 exhibit higher degree of oxidation compared to that synthesized using KMnO4, H2SO4, and NaNO3. Also, they reported that this, often called improved Hummers’ method, does not produce any toxic gases and is suitable for large-scale production of GO.

Recently, some more methods have been reported for synthesizing GO, which include elimination of NaNO3 from reaction mixture or using K2FeO4 instead of KMnO4 or using K2FeO4 + KMnO4. A consolidated list of various techniques reported for the GO synthesis is given in Table . In the past, the electrochemical exfoliation of graphite has been performed to obtain graphene using different types of electrolyte solutions, such as organic/inorganic salts or solvents, bases, strong acids, oxidants, polymers, ionic liquids (sodium tungstate) aqueous solution, tetrasodium pyrophosphate, potassium sulfate, NaOH, sulfuric acid (H2SO4), phosphoric acid (H3PO4), HNO3, HCl, HBr, cetyl trimethylammonium bromide, and protic ionic liquid.[25−34]

Table 1

Comparative Analysis of GO Synthesis Methods Reported in the Literature

reaction time	reaction temperature (°C)	carbon source	oxidizing agents	GO synthesis method	ID/IG ratio	interlayer spacing “d” (nm)	ref
		graphite	N2HO5, KClO3				(32)
		graphite	N2HO5, KClO3				(33)
1.5 h	>35	powder graphite flake	NaNO3, H2SO4, KMnO4	Hummers’			(15)
12 h	>50	graphite flake	H2SO4/H3PO4, KMnO4	improved Hummers’		0.95	(20)
13 h	>35	graphite powder	H2SO4, NaNO3, KMnO4, K3Fe(CN)6	modified Hummers’		0.77	(23)
7.5 days	40	graphite powder	NaNO3, H2SO4, KMnO4	Hummers’ and improved Hummers’ with purification		0.81	(16)
7.5 days	40	graphite powder	H2SO4, KMnO4	improved Hummers’ with purification	0.61	0.831	(17)
5	80	graphite powder	K2S2O8, MnO2, P2O5, and H2SO4	improved synthesis			(19)
24	50	graphite powder	H2SO4, H3PO4, and KMnO4				(21)
5	35	graphite flake	KMnO4, K2FeO4, H3BO3		GO1-1.07	GO1-0.83	(34)
					GO2-0.94	GO2-0.81
6	55–60	graphite electrode	H2SO4, H3PO4, KMnO4	electrochemical followed by oxidation with KMnO4	0.94	0.996	present work

Most of the studies in the literature use GO synthesized by Marcano’s approach using graphite powder as the precursor. The motivation for the present work was to synthesize graphene oxide sheets using graphite electrode as precursor material. The purpose of using electrochemical approach is to exfoliate or to peel off graphite layers from electrode into the reaction mixture and further oxidize the material to synthesize GO sheets. Most of the previously reported synthesis methodologies require longer time, which increases the production cost. This is due to the fact that the slow diffusion of oxidants between the graphene sheets restricts the formation of GO.[20] Present study also aims to minimize the time required for synthesizing GO. It may be noted that the electrochemical approach is easily scalable as well.

In the present study, we report the synthesis of GO from graphite electrode. First, graphite was exfoliated from graphite electrode in acidic mixture (H2SO4 + H3PO4), which caused exfoliation of graphene layer and increased the interlayer spacing between the sheets. In the second step, the exfoliated graphene sheets were oxidized using KMnO4. The obtained GO sheets were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and UV–visible and Raman spectroscopy. The morphological characteristics of the sheets was analyzed by field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) techniques.

Results and Discussion

Structure and Microstructure Studies

The XRD patterns of graphite electrode and exfoliated graphite are shown in Figure a. Pristine graphite exhibits it characteristic diffraction peak at 2θ = 27.6° for the (002) plane, with an interlayer d-spacing of 0.323 nm determined using Bragg’s law equationwhere n = 1 and θ is the Bragg’s angle. In case of exfoliated graphite, diffraction peak for the (002) plane was observed at 2θ = 26.4° for which the interlayer d-spacing was determined as 0.337 nm. After exfoliation of graphene sheets from graphite electrode, an insignificant change in d-spacing was observed. This indicates that only pure graphene sheets are peeled off from electrode without any chemical modification (i.e., oxidation) into the electrolyte solution. The XRD patterns of different batches of GO are shown in Figure b. Results revealed that with increasing reaction times, the (002) peak shifts toward lower 2θ value, suggesting an increase in the interlayer d-spacing. The d-spacing observed for GO (12 h) was determined as 1.046 nm, which is 1.18 times greater than that of GO synthesized using Marcano’s approach. Also, the diffraction peak observed for different GO batches is broader compared to that of precursor material graphite, suggesting deterioration in the crystallinity of the material (Figure b). Therefore, it can be concluded that well-separated GO sheets were obtained using our two-step method.[16,17,20,23,35,36]

Figure 1

XRD patterns of (a) graphite electrode and exfoliated graphite and (b) different GO batches.

Raman spectroscopy is a powerful tool for characterizing graphene and its derivatives. Raman spectra of different GO batches are shown in Figure S1 (Supporting Information). Defect-free graphite exhibits characteristic G-band at 1579 cm–1 in Raman spectroscopy, which corresponds to E2g vibration mode of sp2 hybridized carbon atoms. Upon oxidation from graphite to GO, rupturing of C=C double bond occurs, which results in the generation of sp3 hybridized carbon.[37−39] The presence of sp3 hybridized carbon atoms in GO is reflected by a defect band at 1357 cm–1, which is also known as D-band.[40] The ratio of the area of D-band to G-band, i.e., ID/IG, gives the measure of the disorderness/defects in GO.[41] The area ratios ID/IG for different GO batches are shown in Figure . It should be noteworthy here that with an increase in the oxidation time, ID/IG ratio increases significantly up the oxidation time of 6 h and then decreases. The highest value of the ID/IG ratio of 1.24 was achieved for the GO batch corresponding to 6 h. A further increase in oxidation time resulted in the lowering of ID/IG ratio. The reason for this may be the structural dominance of graphene sheets over the disordered areas, which causes lowering of the intensity of D-band.[42−45] For GO batch prepared using Marcano’s approach, the ID/IG ratio was determined as 1.18, which is lower compared to the ID/IG ratio for the GO (6 h) batch.

Figure 2

Calculated crystallite sizes from Raman and XRD data, and the ID/IG area ratio for GO samples with varying oxidation times and Marcano’s method.

Raman spectroscopy was further used to determine the crystallite size of the as-synthesized GO sheet. The formula used for calculating the crystallite size is given below[41−43]where La is the crystallite size and λ is the wavelength of laser source (514 nm). The crystallite size obtained for different GO batches are shown in Figure . The crystallite size for GO (1 h) batch was determined as 20.9 nm, which reduced to 13.5 nm when reaction was performed for 6 h. This decrease in crystallite size is attributed to the decrease in the GO domains due to the oxidation-induced structural disorder in the form of sp3 hybridized carbon atoms. Also, increase in oxidation time beyond 6 h caused an increase in the crystallite size. The crystallite size of GO prepared using Marcano’s approach is greater than that of GO (6 h) batch, suggesting that more defect levels were induced using our approach compared to Marcano’s approach. The crystallite size of the as-prepared sheets was also determined using the Debye–Scherrer equation (D = 0.94λ/(β cos θ), where λ is the X-ray wavelength (0.15404 nm), β is the full width at half-maximum, and θ is the diffraction angle). The crystallite sizes from XRD data of different GO batches (Figure ) were found be in the range of 3.3–4.3 nm. It is seen that there is difference in crystallite size range as calculated from XRD and Raman spectra. This is due to the fact that crystallite size calculated from Raman spectra depends on the excitation wavelength. Raman spectra obtained from lower excitation wavelength is likely to give crystallite size similar to that obtained from XRD data.[43]

Chemical and Molecular Study

The FTIR spectra of pristine graphite are shown in Figure a, which consist of low-intensity peaks at 3434 and 1630 cm–1 for O–H and C=C stretching vibrations modes, respectively. The presence of various functional groups in different GO batches is observed in the FTIR spectra (Figure a). Different batches of GO sheets reveal a peak at 3434 cm–1, which is attributed to the O–H stretching vibration modes of −COOH and C–OH functional groups. Further, all of the GO batches show peaks at 1747 and 1637 cm–1, which are attributed to C=O stretching and C=C bending vibration modes. For other functional groups like C–O–H and C–O–C, peaks appeared at 1403, 1235, and 1045 cm–1.[20,23,46−48]

Figure 3

(a) FTIR plots for GO batches; (b, c) normalized FTIR plot; and (d) RPOCFG profile for GO samples with varying oxidation times and Marcano’s method (M).

A more detailed study of the structures of the as-prepared GO batches was obtained by plotting normalized FTIR plots (according to the procedure reported by Guerrero-Contreras and Caballero-Briones),[43] which are shown in Figure b,c. The normalized FTIR data of various GO batches revealed prominent peaks at 1724, 1622, 1407, 1226, and 1044 cm–1 for C=O stretch in carboxylic, C=C aromatic stretch, C–O–C ether, C–OH stretch in acids, and C–O–C epoxy groups, respectively. The intensity of various peaks increased with an increase in reaction time and attains maxima for GO (6 h). The intensity of peaks for oxygen-containing functional groups was found to decrease at higher reaction times (>6 h). It should be noteworthy here that the intensities of various peaks in GO prepared by Marcano’s approach are significantly lower compared to those of GO (6 h), suggesting higher concentration of oxygen-containing functional groups in GO (6 h). The deconvoluted FTIR spectra for the range of 1500–1850 cm–1 for all GO batches is given in Supporting Information Figure S2. The deconvoluted spectrum of GO (1 h) showed a single peak for aromatic C=C stretch at 1622 cm–1. However, a new peak was observed at 1580 cm–1 for batches corresponding to 2 h or more, which is again assigned to C=C stretching vibrational mode of substituted aromatic rings in GO. It was also observed that the intensity of this peak increased with time, indicating an increase in the concentration of substituted aromatic ring in the samples. This increase is due to introduction of various functional group in the graphene network. The maximum intensity of this peak was achieved for the GO (6 h) batch, suggesting the presence of higher concentration of functional group compared to other batches.

The degree of oxidation of different GO batches was further evaluated by calculating the relative percentage of oxygen-containing functional groups (RPOCFG) with respect to the presence of all functional groups observed in the wavenumber range of 900–1850 cm–1 (for all peaks in Figure b,c). RPOCFG was calculated using the following formula[43]Figure d presents the value of RPOCFG in different GO batches. It was observed that the value of RPOCFG increased with an increase in the time of oxidation. The highest value of RPOCFG of 68.1% was achieved for GO (6 h) batch, which reflects the highest concentration of oxygen-containing functional groups in the sample. For other GO samples, the RPOCFG values were lower. This indicates that the lower oxidation degree was achieved for these batches. Also, GO prepared using Marcano’s approach gave RPOCFG < 68.1%.

XPS analysis, shown in Figure , further reveals the chemical composition and binding states of carbon and oxygen in GO (6 h). The XPS images of GO (6 h) are mainly composed of C 1s and O 1s. Figure a shows the deconvoluted spectra of C 1s, in which three characteristic peaks are fitted at 284.38, 286.69, and 288.54 eV for C–C/C=C bond (sp2 and sp3 structure), C–OH/C–OC bond, and O=C–OH groups, respectively.[49]Figure b shows the deconvoluted spectra of O 1s, which also shows the presence of three characteristics peaks at 531.44, 532.81, and 533.74 eV corresponding to C=O, C–OH, and adsorbed water molecules.[50,51] The XPS images of the GO (6 h) batch revealed peaks corresponding to C 1s and O 1s only, suggesting the absence of any other impurity in the prepared sample. Further, the degree of oxidation achieved was explain in terms of O/C ratio, which is determined as 0.531.

Figure 4

(a) XPS (a) C 1s and (b) O 1s spectra of GO (6 h).

Boehm titration is a qualitative as well as quantitative tool for the determination of various oxygen-containing functional groups, such as carboxylic acid, phenols, and lactones. This method is based on the difference in the acidities of the above-mentioned functional groups. The concentration of various types of acidic functional groups is calculated under the assumptions that NaHCO3 neutralizes carboxylic groups, Na2CO3 neutralizes carboxylic acid and lactone, and NaOH neutralizes all carboxylic, lactonic, and phenolic groups. The results of Boehm titration are shown in Figure . It is observed that the concentration of phenolic group is maximum in the GO (1 h) batch and minimum in the GO (6 h) batch. On the other hand, the concentration of carboxylic and lactonic groups increases in a regular manner with an increase in reaction time. This increase in carboxylic groups is attributed to the oxidation of some of the hydroxyl groups to carbonyl groups and then to carboxylic groups during the course of oxidation reaction. Similarly, increase in lactonic functional groups is due to the loss of water molecules from the adjacent carboxylic and phenol groups (at the edge of GO sheets). The proposed oxidation mechanism corroborates well with the available literature.[12,52]

Figure 5

Concentrations of various functional groups in GO samples with varying oxidation times and Marcano’s method.

UV–Visible Studies

The structural modification of graphite by increasing the degree of oxidation so as to convert it into GO is also evident from the UV–visible spectra shown in Figure . Dispersions of different GO batches were prepared in deionized water by ultrasonication and their absorption spectra were recorded. It was observed that all GO batches revealed a sharp absorption peak at ≈230 nm, which corresponds to the electronic excitation form π → π* molecular orbital of the aromatic sp2 domains, i.e., C=C bonds. In addition, a weak shoulder band was also observed at ≈300 nm, which was associated with the n → π* electronic transitions due to various oxygen-containing functional groups, such as −COOH, −CHO, and C–O–C. The increase in the intensity at 300 nm is attributed to the presence of highly oxidized graphene basal plane. This produces a greater amount of isolated aromatic rings (or lesser amount of extended conjugated aromatic rings) in the prepared GO batches, which in turn increases the intensity of C=C bonding peak at ≈230 nm.[50,53,54] Similar results were also reported in the literature.[16,42,55]

Figure 6

UV–visible spectra of different GO batches.

Morphological Studies

The morphologies of different GO batches were analyzed by FE-SEM, and the results are presented in Figure a–g. The SEM images of the GO sheets corresponding to 1 and 2 h (Figure a,b) reveal the formation of poorly exfoliated wrinkled structures. It may be noted that the lateral dimensions of sheets are of the order of 10 μm or greater and fit the definition of large GO sheets.[56] From energy-dispersive X-ray (EDX) measurement, the C/O ratios for the GO (1 h), GO (2 h), and GO (4 h) batches were determined as 1.13, 1.15, and 1.17, respectively. On the other hand, GO (6 h), GO (8 h), GO (10 h), and GO (12 h) showed the formation of well-exfoliated sheet-like morphology with curled edges. The C/O ratios for these batches were found to be 0.94, 1.14, 1.21, and 1.28, respectively. The TEM images of GO (6 h) at two different magnifications (Figure h,i) also reveal the formation of a thin sheet. The crystallinity of GO sheets was also investigated by the selected area electron diffraction (SAED) technique (Figure j), which showed the presence of diffused concentric diffraction rings indicating low crystallinity of the material.[4,20,23] This observation is well corroborated with the result of XRD. Hence, it can be concluded that the oxidation of graphite to graphene oxide is accompanied with the worsening of the crystalline property of the material. The semi-amorphous nature of GO has been reported by a number of investigators.[57,58]

Figure 7

FE-SEM images of GO corresponding to reaction times of (a) GO (1 h), (b) GO (2 h), (c) GO (4 h), (d) GO (6 h), (e) GO (8 h), (f) GO (10 h), and (g) GO (12 h); (h, i) TEM images of GO (6 h); and (j) SAED pattern of GO (6 h).

Thermogravimetric Studies

Thermogravimetric analysis (TGA) technique was further used as a tool to analyze the degree of oxidation and thermal stability of the as-synthesized GO batches. TG curves of different GO batches are shown in Figure . GO batches obtained for various reaction times revealed ca. 17–23% weight loss in the temperature range of 25–150 °C due to the removal of physically adsorbed and intercalated water molecules from the sheets. At 150 °C, sudden weight loss was observed for different GO batches, which could be due to the removal of labile oxygen-containing functional groups, such as epoxy, hydroxyl, carboxylate, anhydride, or lactone, from the surface of GO sheets. This indicates the presence of a large number of oxygen-containing functional groups at the edges and in the basal plane of graphene layers. This matches well with the FTIR and Boehm studies, which showed the presence of a high amount of carboxylic and lactonic groups in the GO samples obtained after oxidation for 6 h or longer. Moreover, EDX studies showed higher C/O ratio in these GO samples with oxidation time of 6 h or longer. Also, weight loss at 150 °C is more for the GO (6 h), GO (8 h), GO (10 h), and GO (12 h) batches compared to the GO (1 h), GO (2 h), and GO (4 h) batches. This indicates that with an increase in oxidation time, more oxygen-containing functional groups get introduced in the graphene skeleton, which makes GO thermally less stable. Weight loss in the range of 40–500 °C is attributed to the removal of more stable oxygen groups such as phenol and carbonyl. At temperatures >500 °C, weight loss is due to pyrolysis of GO. Overall, findings of TGA match well with those of FTIR and EDX studies, revealing high concentration of oxygen-bearing functional groups, which are responsible for the low thermal stability of the prepared GO samples.[16,20,23,59]

Figure 8

TG profiles of different GO batches.

Reaction Mechanism

The schematic representation of the method of GO synthesis using our two-step approach is shown in Figure . Briefly, graphite layers got exfoliated from the graphite electrode during the electrochemical step and formed graphite intercalated compound upon insertion of ionic species, such as HSO4–, SO42–, and H+ ions, from H2SO4. In the second step, KMnO4 was added to the reaction mixture, which formed highly reactive dimanganese heptoxide (Mn2O7) species upon reaction with H2SO4.[60,61] Mn2O7 oxidized the defective sites over aromatic double bonds present in graphene skeleton. The reaction of defect centers with Mn2O7 introduced functional groups, which upon hydration with water yield −OH groups. During the course of reaction, loss of water molecule from two −OH groups occurs to from epoxy linkages. Further, sulfonic acid (−RSO3) groups are attached to the edges of the graphene structure, which upon hydration get converted into carboxylic groups.[60]

Figure 9

Graphical representation of the synthesis of graphene oxide using electrochemical exfoliation coupled with oxidation.

Conclusions

Graphene oxide sheets were successfully synthesized through a very simple electrochemical exfoliation of graphite followed by oxidation with KMnO4. The sheets were further characterized using different spectroscopy and microscopy techniques. It was found from XRD, Raman, and FTIR studies that the maximum degree of oxidation was achieved for the GO (6 h) batch. The interlayer spacing between the graphene sheets in GO (6 h) was determined as 0.995 nm with an ID/IG ratio of 1.24. The relative percentage of oxygen-containing functional groups in GO (6 h) as calculated from normalized FTIR plots was found to be the highest (68.1%). XPS and Boehm titration methods were further used to determine the binding states and concentration of various oxygen-containing functional groups in GO batches. UV–visible spectra of different batches of GO revealed a sharp intensity peak at ∼230 nm and a weak adjacent band at ∼300 nm. A detailed characterization of various GO batches reveal the formation of highly exfoliated and oxidized GO sheets with wrinkled surface and curled edges after 6 h of oxidation.

Materials and Methods

Materials

Graphite electrodes with dimensions of 10 cm × 1.5 cm × 0.5 cm were obtained from Graphite India Limited, Delhi. Graphite flakes with a mesh size of ∼20 μm were obtained from Sigma-Aldrich, India. Sulfuric acid (H2SO4, 98%), hydrochloric acid (HCl), diethyl ether (C4H10O), and hydrogen peroxide (H2O2, 30%) were procured from HiMedia India Pvt. Ltd. Potassium permanganate (KMnO4), orthophosphoric acid (H3PO4), and ethanol (C2H5OH) were procured from Merck, India. All reagents were of analytical grade and used without purification.

Synthesis of GO Sheets

Synthesis of GO from graphite electrode was done in two steps: (i) exfoliation of graphene from graphite electrodes so as to peel off/scratch the graphene sheets from graphite electrode with the help of applied current and (ii) oxidation of graphene into GO. In the first step, two graphite electrodes (separated by a distance of 1.5 cm) were employed as anode and cathode. They were dipped in a 66 mL mixture of H2SO4 and H3PO4 with a volume ratio of 9:1. A static current of 1.00 ± 0.1 A, optimized separately, was passed through the electrodes for 4 min using a direct current power supply, which caused exfoliation of graphite from the anode. The amount of exfoliated graphite was determined to be in the range of 0.5–0.6 g.

Thereafter, electrodes were withdrawn from the solution. In the second step, 3 g of KMnO4 was added into the reaction mixture containing exfoliated graphite. The reaction mixture was stirred at 55–60 °C using a water bath for different oxidation time (1, 2, 4, 6, 8, 10, and 12 h) to understand the effect of time on the levels of oxidation. Thereafter, the reaction mixture was cooled to room temperature and the oxidation reaction was quenched by adding 70 mL of ice water containing 2 mL of H2O2. A brownish golden solid material synthesized up to this time was separated by centrifugation and subsequently washed three times with deionized water to remove any of the unreacted material. Further, the solid material was washed with 50 mL of 30% hydrochloric acid (HCl) and 30 mL of ethanol and finally coagulated with diethyl ether. During this process, coagulation of the bunches of sheets takes place. These bunches were separated into sheets (with five to seven layers each) by sonicating the obtained material in water using a probe sonicator for 1 h and then drying at 60 °C in a Petri dish. Different GO batches were referred to as GO (1 h), GO (2 h), GO (4 h), GO (6 h), GO (8 h), GO (10 h), and GO (12 h), where the text in parentheses represents the time of oxidation. For comparison, GO was also synthesized by widely used Marcano’s method using graphite powder as a precursor according to the method developed earlier by performing oxidation for 12 h[20] and further characterized by different methods. The schematic representation of the method of synthesis is given in Figure .

Figure 10

Representation of various steps involved in the synthesis of graphene oxide.

Characterization Techniques

Powder X-ray diffraction patterns of prepared samples were recorded by a Bruker ARS D8 Advance diffractometer in the angular range of 5–90° with a scan rate of 2° min–1 and using Cu Kα (λ = 0.15404 nm) as X-ray source, operated at 40 kV. Raman spectra of the samples were obtained by an inVia Raman microscope (Renishaw) using a monochromatic laser source of wavelength λ = 514 nm. The absorption spectra of the GO samples were obtained using a Shimadzu UV-1800 spectrophotometer in the wavelength range of 200–800 nm. Fourier transform infrared (FTIR) spectroscopy of GO was performed using a Nicolet Nexus instrument to identify the functional groups present in different samples of GO. KBr pellets of the dried samples were scanned in the wavenumber range of 4000–400 cm–1 so as to obtain transmittance (%) versus wavenumber graphs. Transmission electron microscope and field emission scanning electron microscope (FE-SEM) coupled with energy-dispersive X-ray (EDX) systems were used for the analysis of the morphological characteristics of the as-synthesized GO sheets. The transmission electron microscope was operated at 200 kV, and the field emission scanning electron microscope/energy-dispersive X-ray device was operated at 15 kV. XPS analysis was conducted using PHI 5000 versa probe III.

Analysis of Functional Moieties

The Boehm titration method was used for analyzing the concentration and nature of functional groups that are introduced in the graphene skeleton during oxidation. Briefly, 0.1 g of GO sheets was dispersed in 100 mL of 0.1 M NaOH, NaHCO3, and Na2CO3 separately. The obtained dispersion was mechanically stirred for 48 h at room temperature and then filtered to separate GO sheets from solution. Thereafter, 5 mL extracts of NaOH, NaHCO3, and Na2CO3 were titrated with 0.05 M HCl using phenolphthalein as indicator. A controlled experiment without GO sheets was also performed for proper calculations.